The latency protein LANA2 from Kaposi's sarcoma-associated herpesvirus inhibits apoptosis induced by dsRNA-activated protein kinase but not RNase L activation

Abstract

Kaposi's sarcoma-associated herpesvirus (KSHV) uses several strategies to counteract the interferon (IFN) system. In this study, the relationship of the protein LANA2 from KSHV to the IFN-activated protein kinase (PKR) and 2-5A system was analysed. It was found that LANA2 could not abrogate apoptosis or RNA degradation mediated by the 2-5A system. However, expression of LANA2 inhibited apoptosis triggered by PKR. LANA2 also counteracted the PKR-mediated inhibition of protein synthesis and partially blocked PKR-induced phosphorylation of eIF-2a. Analysis of PKR-induced activation of caspases 3 and 9 revealed that LANA2 abrogated activation of caspase 3 but not of caspase 9. These findings show that LANA2 is able to interfere with downstream events triggered by PKR. Hence, LANA2 should be considered as a KSHV defence protein against IFN. INTRODUCTIONVirus infection induces expression of a group of proteins, the interferons (IFNs), which play major roles as a first-line host defence response against pathogens. IFNs are multifunctional cytokines with important roles in the induction of antiviral, cell growth, differentiation and immunomodulatory functions (Harada et al., 1998;Stark et al., 1998). There are two types of IFN, type I (IFN-a/b) and type II (IFN-c). Type I IFNs are produced by virus-infected cells and constitute the primary response against virus infection, whereas type II IFN is a Th1 cytokine produced by activated T cells and natural killer cells and is involved in immune regulation (Vilcek & Sen, 1996;Stark et al., 1998). Induction of IFN is highly regulated through IFN regulatory factors (IRFs), a family of transcription factors with a broad range of activities (Nguyen et al., 1997;Mamane et al., 1999;Taniguchi et al., 2001). The different biological actions of IFN are mediated by the products of specifically induced cellular genes in target cells. Two of these IFN-regulated cellular products are the dsRNA-activated protein kinase (PKR) and the 2-5A system. PKR controls cell growth (Chong et al., 1992), cell differentiation (Petryshyn et al., 1984), virus clearance (Lee & Esteban, 1993;Lee et al., 1996) and induction of apoptosis (Lee & Esteban, 1994). PKR is a serine/threonine kinase activated by dsRNA binding and exerts its effect on subsequent autophosphorylation. Some targets of the activated PKR are the phosphorylation of the eukaryotic translation initiation factor 2 (eIF-2a) and NFkB activation, events that mediate the induction of apoptosis by PKR. Activation of the caspase 9 pathway together with the fas-associated death domain (FADD)/caspase 8 pathway are also events downstream of PKR-induced apoptosis, although this activation is not required for the induction of apoptosis (Gil et al., 2002). The 2-5A pathway is constituted by the IFN-induced proteins 2-5A synthetase and 2-5A-dependent RNase L, together with 2-5A phosphodiesterase. dsRNA activates the 2-5A synthetases that, in the presence of ATP, synthesize a complex mixture (referred to as 2-5AS) of 2959-triphosphorylated oligoadenylic a...

Impact of Protein Kinase PKR in Cell Biology: from Antiviral to Antiproliferative Action

“…No interaction has been observed between LANA2 and PKR. LANA2 inhibits PKR-induced activation of caspase 3 but not of caspase 9, suggesting that the FADD/ caspase 8 pathway is affected by LANA2 inhibition of PKRinduced apoptosis (93).…”

The dsRNA-dependent protein kinase, PKR and cell death

“…102,103 Other viral proteins shown to inhibit PKR-mediated apoptosis include Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded LANA2, as well as Epstein-Barr virus (EBV)-encoded RNAs (EBERs), which may inhibit PKR by preventing correct activation. [104][105][106] Influenza virus protein NS1 has been similarly reported to inhibit PKR, as have selected hepatitis C virus (HCV), herpes simplex 1 (HSV) and human papilloma virus (HPV) proteins. [107][108][109][110] Certainly, studies using PKR-null mice or MEFs lacking PKR activity have confirmed that this kinase is a key component of the host early defense system, which acts in innate immunity prior to the activation of the IFN system and the acquired immune response.…”

Kaposi's Sarcoma–Associated Herpesvirus Viral IFN Regulatory Factor 3 Stabilizes Hypoxia-Inducible Factor-1α to Induce Vascular Endothelial Growth Factor Expression

“…It has been shown that vIRF3 binds and inhibits p53 transcription factor (25). vIRF3 also inhibits PKR-mediated apoptosis (27) and deregulates IFN-a promoter activity by interacting with cellular IRF3, IRF7, and cyclic AMP-responsive element binding protein-binding protein/ p300 (28,29).…”